In the field of implantable stimulator devices, there are now frequent applications where two or more stimulus pulses are delivered substantially simultaneously, or concurrently, at different sites within a patient's body. Thus, in the fields of neurostimulation and cardiac pacing, for example, the implanted system may be required to generate and deliver such concurrent pulses to two or more positions. As used herein, the terms "concurrent" and "simultaneous" are used to mean either at exactly the same time or substantially the same time, e.g., with some overlap.
A problem that arises in such devices is that of cross-currents from one stimulus channel to another. If, for example, two generators deliver pulses simultaneously, and both generators are referenced to the battery of the implantable device, there may be crosstalk in the form of current flow between the separate sites, as well as current flow through the intended body loads. Such crosstalk can have a very detrimental effect on the stimulus pulse, thereby impairing the ability to properly stimulate. Another detrimental effect is that of increased current flow, and resulting battery drain, which is a serious problem for implanted systems.
A first simple solution to the problem is to have two or more entirely separate generators, powered by separate batteries. This is, of course, very impractical and is not available as a solution for that reason. Another longtime solution has been to use transformer isolation, but that likewise is unacceptable due to the size and expense of one or more transformers. The proposed solution of simply letting each generator float from any reference to the battery at the time of delivering the voltage pulses likewise is not a feasible solution, because of the nature of solid state circuits in use in the output stages. The generators and regulators of a chip such as would be used in a modern implantable device are comprised of FETs or other equivalent semiconductor switch elements which, as is known, must be maintained with the proper bias thereacross. Each FET in an integrated circuit (IC) is biased with respect to system ground, which is tied to the negative terminal of the battery in the illustrative embodiment of this invention. Each FET component has an inherent diode, which is normally back biased when the FET is properly forward biased, such that the diode is not a factor. However, if the required bias, e.g., positive bias, is not maintained across the FET, the diode becomes forward biased, and transforms the FET effectively into a conducting diode. If this situation prevails, the FETs are parasitic transistors, resulting in parasitic crosscurrents. Consequently, any floating arrangement of the output stages must take into account the need to control the state of all voltages developed during stimulation, so as to prevent such inadvertent forward conduction of the diode junctions. There is thus an important need in the field of implantable devices which are battery powered, for enabling controllable generation and delivery of two or more concurrent stimulus pulses to different sites in the patient's body, while avoiding crosstalk current flow and maintaining proper biasing of the FET or other IC elements.